your navigation officer will tell you the exact range at the current speed every time without error... the problem is, your speed fluctuates slightly with every wave you hit.
one second your doing 9 knots, the next your doing 8.3, the next your doing 10 and this speed fluctuation can occur over the course of a few seconds depending on sea state. - especially if you ask him under time compression. which if that is the case you may be asking him your range when your speed has dipped to a more economic level... but as soon as he tells you it speeds back up to a less economic level of fuel consumption.
so what you have to do is ask him about 5 or 6 times in a row and then average the 6 numbers he tells you, because they will all be slightly different.
then and only then will you have a good reliable estimate on the range.
better luck next time